The proposed study responds to PAR 07-143 "Research on Pathways Linking Environments, Behaviors and HIV/AIDS" by using innovative methods to examine the space-time patterns of HIV risk behaviors among injection drug users (IDUs) and HIV prevention services in the San Francisco Bay Area from 1987 to 2005. We propose to model the relationship among neighborhood characteristics, neighborhood HIV prevention services, and individual HIV risk behavior outcomes among IDUs. Research on HIV and illicit substance use has traditionally focused solely on individual-level characteristics that confer risk. Emerging research in the areas of HIV and illicit substance use has identified the importance of considering social environmental factors, including access to health care services, when studying individual health behaviors. The proposed study involves applying newly emergent spatial analytical methods to secondary datasets in order to refine the knowledge regarding how neighborhood characteristics might be associated with access to HIV prevention services and HIV risk among street-based IDUs. Our work will adapt the well-known Behavioral Model of Vulnerable Populations to incorporate a well-developed conceptualization of the spatial interaction between contextual variables and individual behavior. We have individual-level and HIV test data from 35,000 serial cross-sectional interviews among street-based IDUs from the Urban Health Study from 1987 to 2005. Other data to be used include archival data derived from the U.S. Census, the Census Neighborhood Change Database, HIV/AIDS reports, crime reports, and geocoded locations of HIV prevention services (including HIV testing sites, drug treatment programs, and syringe exchange programs). The specific aims include Specific Aim 1: To describe the geographic distribution of neighborhood characteristics, HIV prevention services, and HIV risk behaviors among street-based IDUs in San Francisco Bay Area census tracts over a 19-year period;and Specific Aim 2: To examine the statistical associations among neighborhood characteristics (e.g., neighborhood disadvantage or deprivation), HIV prevention services (e.g., drug treatment, syringe exchange, HIV testing) and HIV risk behaviors of IDUs (e.g., syringe sharing, unprotected sex) in the San Francisco Bay Area. Aim 1 will involve the creation of census-tract-level maps to illustrate the geographic distributions of key variables over time. Aim 2 will examine the cross-sectional and temporal associations among neighborhood characteristics, HIV prevention services, and HIV risk behaviors. Analytical methods will include multilevel selection bias models and spatial regression models. This project is among the first to utilize a rich set of geospatial contextual data in conjunction with a unique serial cross-section of 19 years of individual-level data, using advanced spatial analytic methods to better understand the relationships between contextual variables and HIV risk behaviors among street-based IDUs. PUBLIC HEALTH RELEVANCE: This project is among the first to utilize a rich set of geospatial contextual data in conjunction with a unique serial cross-section of 19 years of individual-level data, using advanced spatial analytic methods to better understand the relationships between contextual variables and HIV risk behaviors among street-based injection drug users (IDUs). This work will advance our methodological understanding of how best to study this population using cutting-edge methods that examine the spatial patterns of HIV risk behaviors and the relationship between neighborhood context and availability of HIV prevention services in metropolitan areas. The findings will benefit policy makers, service providers, and researchers.